Micellar structures of dimeric surfactants with phosphate head groups and wettable spacers: A small-angle neutron scattering study

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Abstract
Dimeric or gemini surfactants consist of two hydrophobic chains and two hydrophilic head groups covalently connected by a hydrophobic or hydrophilic spacer. This paper reports the small-angle neutron scattering (SANS) measurements from aqueous micellar solutions of two different recently developed types of dimeric surfactants: (i) bis-anionic C16H33PO4−−(CH2)m−PO4−C16H33,2Na+ dimeric surfactants composed of phosphate head groups and a hydrophobic polymethylene spacer, referred to as 16−m−16,2Na+, for spacer lengths m=2, 4, 6, and 10, (ii) bis-cationic C16H33N+(CH3)2−CH2−(CH2−O−CH2)p−CH2−N+(CH3)2C16H33,2Br− dimeric surfactants composed of dimethylammonium head groups and a wettable polyethylene oxide spacer, referred to as 16−CH2−p−CH2−16,2Br−, for spacer lengths p=1–3. The micellar structures of these surfactants are compared with the earlier studied bis-cationic C16H33N+(CH3)2−(CH2)m−N+(CH3)2C16H33,2Br− dimeric surfactants composed of dimethylammonium head groups and a hydrophobic polymethylene spacer, referred to as 16−m−16,2Br−. It is found that 16−m−16,2Na+, similar to 16−m−16,2Br−, form various micellar structures depending on the spacer length. Micelles are disklike for m=2, rodlike for m=4, and prolate ellipsoidal for m=6 and 10. The micelles of 16−CH2−p−CH2−16,2Br− are prolate ellipsoidal for all the values of p=1–3. It is also found that micelles of 16−m−16,2Na+ and 16−CH2−p−CH2−16,2Br− are large in comparison to those of 16−m−16,2Br− for similar spacer lengths. This is connected with the fact that both in 16−m−16,2Na+ and 16−CH2−p−CH2−16,2Br−, the head group or the spacer is more hydrated as compared to that in the 16−m−16,2Br−. An increase in the hydration of the spacer or the head group increases the screening of the Coulomb repulsion between the charged head groups. This effect has been found to be more pronounced in the dimeric surfactants having wettable spacers.